The present disclosure relates to a thin film semiconductor device and a method for manufacturing the same, and particularly to a thin film semiconductor device that includes plural elements obtained by crystallizing a semiconductor thin film through irradiation with an energy beam, and a method for manufacturing the same.
In a flat display such as a liquid crystal display, thin film transistors (TFTs) are employed as switching elements for active-matrix display of plural pixels. The kinds of TFT include a TFT having an active region composed of polycrystalline silicon (poly-Si) (poly-Si TFT) and a TFT having an active region composed of non-crystalline silicon (amorphous Si) (a-Si TFT).
Compared with the a-Si TFT, the poly-Si TFT has carrier mobility higher by a factor of about 10 to 100 times, and a smaller degree of deterioration of the on-state current. It follows that the poly-Si TFT has superior characteristics as a switching element.
As a fabrication technique for the poly-Si TFT, there has been developed a so-called low-temperature poly-Si process, in which an amorphous silicon film is turned into a polycrystalline film by using only low-temperature processes at temperatures below about 600° C., for achievement of reduced substrate cost. For example, in a low-temperature poly-Si process employing an excimer laser, an amorphous silicon film is irradiated with pulses of laser light shaped into a line beam. In this irradiation, the irradiation position is so slightly shifted at every pulse irradiation that most parts of adjacent irradiated regions overlap with each other and the same position on the film is irradiated with the laser light pulse 10 to 20 times. This process results in achievement of a polycrystalline film having a crystal grain size uniformed across the entire active region.
As another example of the low-temperature poly-Si process, there has been proposed a method in which a crystallized region is formed by irradiating an amorphous silicon film with continuous laser light obtained from e.g. a harmonic of a YAG laser. During the irradiation, the laser light is moved at a constant speed so that the irradiation energy is equalized. After the forming of the crystallized region, patterning is so carried out that a region free from a crystal grain boundary is used as the active region of a thin film transistor (refer to Japanese Patent Laid-open No. 2003-77834 (in particular, paragraphs 0091, 0092 and 0169)).
Furthermore, sequential lateral solidification (SLS) has been proposed by Columbia University and so forth as a method in which the width of lateral growth of a crystal is defined by multi-step irradiation with use of a mask (refer to A. T. Vouysas, A. Limnov, and J. S. In, “Journal of Applied Physics” (2003), Vol. 94, P. 7445 to 7452).
In recent years, flat panel displays as mentioned above, liquid crystal displays allowing a high frame rate are being developed for further enhancement in moving image properties and contrast properties. In addition, novel displays such as self-luminous displays typified by organic EL displays are also being developed. Along with these developments, there has been an increasing demand for TFTs that suffer no characteristic deterioration even when large current is suddenly applied thereto, and of which characteristic variation is small, as switching elements applicable to these displays.
However, poly-Si TFTs obtained through the above-described existing low-temperature poly-Si process problematically involve larger variation in characteristics among elements, specifically, larger variation in the initial threshold voltage and on-state current in particular, compared with a-Si TFTs, although the poly-Si TFTs have great advantages such as ease of application of comparatively large current thereto, higher carrier mobility, and smaller characteristic deterioration.
In order to reduce this variation, it has been attempted in the above-described crystallization employing an excimer laser to minimize variation among elements by forming a film in which similar crystals with a crystal grain size of about 300 nm, equivalent to the wavelength of the laser light, have been grown. However, even using such a polycrystallized film was not enough to offer a sufficient effect of suppressing the characteristic variation among elements.
This is because in crystallization by a method in related art which employs an excimer laser annealing apparatus, it is difficult to control the size of crystal grains in a poly-Si film with high accuracy and hence uneven crystal grain size is obtained. The crystal grain size unevenness leads to variation in the number of crystal grain boundaries in a channel region among TFTs, which results in variation in characteristics of the TFTs (refer to e.g. K. Yamaguchi; et al; J. Appl. Phys., Vol. 89, No. 1, pp. 590, and M. Kimura et al; JAP. J. APPL. PHYSI. Vol. 40 Part 1 (2001), No. 1). In a display including organic EL elements as its display elements in particular, this problem is important because the variation appears as color unevenness and so on in the display part.
It is difficult even for the low-temperature poly-Si process described in Japanese Patent Laid-open No. 2003-77834 (in particular, paragraphs 0091, 0092 and 0169) to sufficiently suppress the variation in characteristics of TFTs. This would be because each crystal region inside a channel region becomes large and therefore influence dependent upon the presence or absence of defects, dislocations and so on inside a crystal is greatly reflected in the characteristic variation. Furthermore, FIG. 8 in A. T. Vouysas, A. Limnov, and J. S. Im, “Journal of Applied Physics” (2003), Vol. 94, P. 7445 to 7452 shows that TFTs formed by the SLS method involves mobility variation larger than 10% even when the TFTs have been formed through the optimum process. This would be due to the existence of a myriad of uncontrolled crystal grain boundaries in a crystal region in a laterally grown part.